Postgresql on SAN

Hi ppl,
What need to do (understand, to devel) to allow several
postgres instance  running from several server to access to the
same data (no replication at all) hosted on a SAN ?
I'm probably wrong but i think this type of dev should
be easier to realize than replication ? Because all node are
always consistent since they use exactly the same data ? 
In fact i'm asking why opensource db go only to the
replication solution than "true" clustering solution using SAN
like Oracle RAC ? I don't see the big advantage of the replication
method (of course, for application which need more than few nodes
hosting "small" db). The price could be an answer but what about a db of
several hundred giga byte ? When a node is added it must contain this
capacity nearly for "nothing" (i thinks its more and more a waste when
number of node grow ...).
Is this a planed feature ? 
Sincerely, Yannick.